How I know? Look here. I found these today. What is amazing to me is that none of the birds or creatures - whatever they may be - is trying to defend the nests. Actually it seems as if they are put in my way deliberately.
Just look how different they are. Some are almost flat, some are real thick, some have one really big egg and now there is one with three tiny eggs.
I have looked around for suspicious signs today, but I found nothing that couldn't have been done by the cats. There is another thing - if there are birds around, why are the cats so cool about it? Does it mean something that sometimes they sit there and stare up into a corner that seems totally empty to me?
Instead of getting closer to a solution it seems the matter is getting more complicated.